The invention concerns a linear motor of the synchronous brushless type excited by permanent magnets.
To be more exact, the invention concerns a linear motor of the type comprising a mobile armature, slider or cursor, equipped with a plurality of compartments inside which respective electric coils are stably housed, and one fixed bar on which a plurality of permanent magnets are mounted and arranged, during use, facing the electric coils.
The mobile armature of the motor is made of aluminium or its alloys or of ceramic material, and is suitable to house also a ferromagnetic bar which cooperates with the coils to close the magnetic circuit.
The invention also concerns the method to manufacture the linear motor.
The state of the art, as disclosed for example in GB-A-2.352.954, includes synchronous brushless linear electric motors of the general type as described above. Such motors comprise a mobile part which normally consists of an armature or frame on which the housing compartments are made, normally equidistant, inside which electric coils associated with feed means are inserted and clamped.
A relative fixed bar, normally made of ferromagnetic material, is present in a position facing at least one side of the armature; a plurality of permanent magnets are mounted on the fixed bar, arranged aligned and usually equidistant in the direction of movement of the armature which carries the electric coils. The magnets have alternating polarities.
In other constructional embodiments, the motor can have the fixed part comprising the electric coils and the mobile part comprising the permanent magnets.
The armature where the coils are housed and the ferromagnetic bar on which the permanent magnets are mounted are separated from each other by an air interspace.
The working principle of linear motors of this type exploits the force of repulsion which is created by sequentially inverting the direction of circulation of the electric current circulating in a coil every time the coil moves from a position facing a magnet with a certain polarity, for example, positive, to a position facing a magnet with a negative polarity.
In conventional motors (see for example U.S. Pat. No. 6,140,734) the coils are buried in an insulating material, for example resin, inside the respective housing compartment of the armature, and are cemented in the furnace by means of heat treatment which causes the resin to be activated (melted) and penetrate between the spirals of the coil. The insulating material is necessary to eliminate phenomena of magnetic friction between adjacent coils which cause a deterioration to the performance of the motor.
The insulating material, having set between the spirals of the coil, also acts as a mechanical support for the stable accommodation of the coils in the relative compartments of the armature, in order to ensure a precise positioning with respect to the fixed magnets.
However, it has been found that using a hot cementing process on insulating material causes a lack of mechanical rigidity due to the interstices between the spirals which are not completely filled, particularly in the inner compartment of the coils. When the motor is used at high frequency conditions, in the long term mechanical stresses are created on the coil which lead to a loosening of the spirals which are thus exposed to the environment, with negative repercussions on the functioning and efficiency of the motor.
The presence of interstices between the spirals causes a deterioration in the interaction conditions of the magnetic fields produced respectively by the current circulating in the coils, and by the permanent magnets, with a reduction in the value of the force of repulsion which drives the motor.
Moreover, the presence of insulating material functioning as a mechanical support for the coils determines a low capacity to dissipate the heat generated by the Joule effect, with consequent problems of overheating in the armature of the coils.
The present Applicant has devised and embodied this invention to solve the shortcomings of the state of the art, and to obtain further advantages.
The invention is set forth and characterized essentially in the respective main claims, while the dependent claims describe other innovative characteristics of the invention.
The purpose of the invention is to achieve a brushless linear motor of the synchronous type, with permanent magnets, which has improved characteristics of mechanical resistance to stresses, heat dissipation, structural stability of the spirals buried in the armature, smaller overall bulk, efficiency and speed of response.
Another purpose is to obtain a low-cost and low-weight linear motor which can in any case guarantee a sufficiently high motive power substantially for all the applications where it can be used.
In accordance with these purposes, according to a first characteristic, the linear motor according to the invention comprises a structural armature made mainly of aluminium, or alloys thereof, in which the compartments are made to stably house a plurality of mating coils associated with electric feed means and aligned substantially along the direction of motion.
According to another characteristic, the structural armature is made mainly of ceramic material.
Using a structural armature made mainly of aluminium, or alloys thereof, or ceramic material, allows to guarantee great mechanical rigidity while keeping the overall weight limited, even in conditions of great stress, for example deriving from a prolonged use of the motor at high frequencies.
Moreover, the structural armature made of aluminium or ceramic ensures a great capacity of heat dissipation, which prevents possible overheating deriving from a prolonged circulation of electric current in the spirals of the coils.
The linear motor according to the invention also comprises one fixed bar only, on which are mounted, in a position facing the coils of the mobile armature, a plurality of permanent magnets suitably distanced and with reciprocally alternating polarity.
The presence of a single fixed bar of magnets allows a considerable saving in terms of cost and weight of the motor, and considerable advantages in terms of improved heat dissipation. The overall motive force is slightly reduced with respect to a traditional motor with two rows of magnets, but remains in any case high enough substantially for all cases of possible application.
Behind the coils, with respect to the side facing the magnets, a bar made of ferromagnetic material is mounted on the armature, by means of which bar the magnetic circuit established between the coils and the magnets is closed.
According to another characteristic of the invention, the structural armature made of aluminium or ceramic has, in cooperation with at least a segment of the perimeter surrounding said housing compartments of the coils, interruptions which define electric discontinuities and are able to prevent the electric circuit from closing on the armature itself; these interruptions are also able to avoid that the currents induced on the armature flow freely along the armature itself and disturb the operation of the motor.
In a first embodiment the interruptions or discontinuities are made on a lateral segment of the armature which separates two adjacent housing compartments of the relative coils. According to another embodiment, the interruptions or discontinuities are made in cooperation with an upper or lower segment of the relative housing compartment.
According to a preferential embodiment of the invention, the aluminium or ceramic structural armature which defines the housing compartments of the coils comprises at least a module able to define casting fissures inside which an insulating material is poured in the liquid or melted state, for example polymerizable resin or other material comparable therewith.
During the casting, the insulating material penetrates in depth until all the interstices between the spirals of the coils, and also the inner compartment of the coils, are filled. The resin sets and stabilizes, by means of polymerization, forming a stable whole with the relative coils; this whole guarantees absolute mechanical stability, electric insulation, good capacity of heat dissipation and that there are no empty spaces where electricity cannot be conducted.
According to a variant, on a plane transverse to the direction of advance of the mobile armature, the motor comprises two coils, or multiples of two, adjacent and substantially parallel, made by means of a single continuous winding.
Using a single winding to form two adjacent coils, the two ends of the conductor, connected to the source of feed to form the electric circuit, always emerge from the outer periphery, respectively of one and the other coil, remaining contained inside the lateral bulk of the coil itself. With this configuration there are no lateral thicker parts formed due to the end of the conductor emerging from the armature, which thicker parts can compromise an efficient filling of the empty spaces by the insulating material and hence create problems of correct electric conduction, heat dissipation and mechanical stability.
In a preferential embodiment, the armature comprises at least a module, comparable from the electric point of view with a single coil, configured dimensionally so as to contain two coils, or multiples of two, or pairs of coils, arranged aligned in the direction of motion.
According to a variant, the module is suitable to contain three coils, or multiples of three, or pairs of coils arranged aligned in the direction of motion.
The module has a size, in the direction of movement, such as to cover an even number of magnets so that, during the movement of the mobile part of the motor, the sign of the magnet which is left is always equal to the sign of the new magnet which is covered by the armature which bears the coils. In this way, the induced currents which are generated between adjacent magnets and which close on the metal armature are mutually cancelled and therefore do not oppose, or in any case create interference with, the motion and/or the performance of the motor.